A narrower follow-up, since the general answer is now clear:

How would you tell the difference between that and the alternative explanation?

anders_CPH said:

Give anything pharmacological four weeks before you judge it, and give anything measured weekly a four-point rolling average before you call it a…

I read this differently. The confidence in this thread is running ahead of the evidence, and I would rather the uncertainty were stated than smoothed over because it is unsatisfying.

anders_CPH said:

Give anything pharmacological four weeks before you judge it, and give anything measured weekly a four-point rolling average before you call it a…

Agreed, and the enforcement dates were staggered by category — 503A first, 503B a few weeks later — because outsourcing facilities have manufactured inventory and clinic contracts to unwind while a 503A makes to order.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

anders_CPH said:

Give anything pharmacological four weeks before you judge it, and give anything measured weekly a four-point rolling average before you call it a…

Saving this. It is the first explanation that did not require me to already understand it. Adding it to my notes with a link back to this thread.

Closing the loop on my own question.

The bulks-list asymmetry was the piece I had missed entirely. It explains why one of my two pharmacies is still arguing it can supply and the other simply stopped.